This invention relates to calibration of an analog-to-digital converter, and especially to calibration of third and higher order integration periods of a multi-slope analog-to-digital (A-D) converter.
In A-D conversion, a multi-slope type A-D converter is frequently used for increasing conversion speed and resolution. A simple type of the multi-slope A-D converter is a triple slope type A-D converter. A voltage-to-time diagram of the triple slope A-D converter is shown in FIG. 1. During a first integration period T.sub.1 an input analog signal is integrated, to generate a rising slope at the output of the integrator which begins at t.sub.0 and ends at t.sub.1.
At time t.sub.1 a second integration period T.sub.2 begins, and a first reference current of opposite polarity to the input signal is supplied to the integrator, so that the integrated signal begins to fall linearly at time t.sub.1 as shown in FIG. 1A. When the integrated signal reaches a predetermined reference level at time t.sub.2, in this case zero voltage, the supplying of the first reference current to the integrator is stopped in synchronism with the first falling edge of a clock signal after t.sub.2, that is at time t.sub.5.
After the second integration period T.sub.2, a third integration period T.sub.3 begins, during which a second reference current is integrated for improving conversion resolution. The second reference current has opposite polarity to the first reference current and also has a smaller magnitude than the first reference current. Thus during the third integration period T.sub.3, the integrated signal has a rising slope at a relatively smaller angle as compared with that of the second integration period T.sub.2. The third integration period T.sub.3 ends at time t.sub.4 when the integrated signal reaches the reference potential. By measuring the second integration period T.sub.2 and the third integration period T.sub.3 by counting clock signals provided during those periods, a digital signal corresponding to the input analog signal is obtained.
Since the second reference current may be selected smaller than the first reference current, for example by a factor of 1000, the slope of the third integration period may be made 1/1000th of the slope of the second integration period, and thus the resolution of the third integration period increases by 1000 times over that of the second integration period. Thus the A-D conversion resolution may be increased by employing a plurality of reference currents.
For converting an analog signal to a corresponding digital signal accurately in a multi-slope A-D converter, the first reference current should be adjusted correctly and it should be stable enough to maintain its accuracy for the period of measurement. Further, for achieving conversion accuracy, the ratios of the first, second, third, etc., reference currents should be correct and stable. A deviation from the correct ratio causes error when counting during the third or higher order integration periods because the digital data is calculated on the assumption that the ratios are correct.
In the prior art those ratios are adjusted manually, for example, by variable resistors in the reference current generators. Thus it is time consuming to adjust these ratios to be strictly correct. Also in prior art, for keeping those ratios within a high range of accuracy, it is necessary to use expensive parts and circuits.